
\subsubsection{Antenna}
The first stage of the eavesdropper is the antenna. The antenna will be a loop antenna. The reason for this is that the antenna has to capture the magnetic field lines of the two inductors. So basically the antenna is an inductor. This is also the reason that a loop antenna is frequently called a magnetic loop. The loop is circular because it is the most efficient in creating a large antenna surface with the least amount of material. The challenge is that it has to capture enough magnetic field lines at larger distances. And it becomes much harder at larger distances because the magnetic field strength falls of with $r^{-3}$ in the near field region \cite{MagneticFieldArea}. This is also confirmed with measurements from the antenna. The measurement results are in the appendix \ref{appendix:antenna}. The antenna can be seen in \cref{Built antenna}.
\begin{figure}[ht!]
	\centering
	\includegraphics[width=100mm]{Assets/antennapic.png}
	\caption{Built antenna}
	\label{Built antenna}
\end{figure}

A loop antenna has a certain efficiency. An efficiency of $20\%$ means that $20\%$ of the available power in the field is received, and is thus not lost due for example reflection, conduction, and dielectric losses.
Now the efficiency of the loop antenna can be mainly increased by the amount of loops, the surface, the type material, and thickness of the material. In the appendix \ref{appendix:antenna} the exact calculation can be seen. Here, it can also be seen that adding more loops or just increasing the surface does not really matter, the effect is the same. Increasing the wire radius does matter, in principle the bigger the better. For the antenna a tube is used, this is done because a tube has a lot of copper and its also a sturdy construction. The amount of copper is approximately equal to a $3mm$ radius wire.  

The reason that the antenna is 1 loop, is that some unwanted things happen with more loops. The first unwanted effect is the proximity effect that will start to play a role. This effect decreases the efficiency of the antenna, and is explained in \cref{appendix:antenna}. Now as can be seen in the appendix this effect can be made minimal if the space between the windings is big enough, relative to the radius of the antenna. So this is not the main reason for choosing only one loop.

A loop antenna needs to be tuned to the required frequency, which is called the resonance frequency. Tuning can be done by adding a parallel capacitor. In appendix \ref{appendix:antenna} this is explained. 
There are also some example calculations in the appendix. It can be seen that these calculations lead to a capacitance between $10pF$ and $100pF$. A high inductor value will give low capacitor values, and vice versa. Now a capacitance around $10pF$ - $30pF$ will give problems, due parasitic capacitance in the rest of the circuit; very small changes in capacitance will ruin the resonance frequency. Tuning to the right frequency is extremely important, because a small change can make the signal twice as strong. And not tuning at all, will give no signal at all. So to prevent this, the inductance may not become too high.

The amount of inductance is dependent on the diameter, width and number of windings. The calculation is in \cref{appendix:antenna}. It can be seen in the appendix that the inductance is rising faster due to adding windings then due to increasing the diameter. As said before, for the efficiency it does not matter which one is increased, but it does matter for the tuning of the resonance frequency.This is the main reason for using only one loop. An antenna with only 1 loop can be more efficient due the fact that is stays tunable. 

The last part which is important for the design of the antenna is the bandwidth. As already said the signal that the antenna has to receive has a bandwidth of around $1$ MHZ. In \cref{appendix:antenna} it is explained how to achieve this. 

In the final product a tunable capacitor and a tunable resistor is placed. This is done because the resonance frequency of the antenna can change because of a change in the environment. It could also be useful to be able to decrease and increase the bandwidth, for tuning the overall circuit. So in the end its actually just an RLC circuit, see \cref{RLC Circuit}. The antenna had a final inductance of around $1.5\mu{}H$.

\begin{figure}[ht!]
	\centering
	\includegraphics[width=50mm]{Assets/RLC.png}
	\caption{RLC Circuit}
	\label{RLC Circuit}
\end{figure}

Improvements could be building an antenna which would be more efficient but still tunable. The best way do to this is increasing the wire radius. This will cause the antenna to still be tunable, but its efficiency will go up. Another way to do this is increasing the radius of the antenna, but this will give a very big antenna, which is not really wanted. Further more, soldering wires to a copper tube is not doable. This problem is solved by using hose clamps, but this probably gives a lot of parasitic capacitance and this is thus not very handy for the tuning part. This could probably be solved by using something that can become way hotter, like a welder. Also the connection with a coaxial cable will add capacitance. This should be taken into account. 